PAINFUL LESSONS LEARNED
Judith T. Duff
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In 1997 I attended a workshop with John Neely and Owen Rye at Utah State University. This incredible three-week workshop was my first participation in a wood firing. The group started on a Monday mixing various clay recipes and began the first firing in the Train kiln on Thursday with bisqued pots.  The days continued at this pace and by the end of the workshop, we had fired five times using the Train and the catanary arch kiln. The effects from the Train kiln were especially exciting to me. I was drawn to the beautiful build up of natural ash rolling around the surfaces of the tumble-stacked pots! This experience changed my life and my pots. When I returned home I built my train kiln by counting brick on slides I had taken during the workshop. Now, after 43 firings my stacking and firing techniques have evolved to the point that I get fairly predictable results. Included in this article is a summary of how I now stack and fire my kiln, but first here are a few of the “Painful Lessons Learned”.

The first firing resulted in a multitude of mistakes that taught me many lessons. I had, what I thought, were very detailed notes from the Utah workshop on the stacking and firing. Not really knowing much about wood firing, I soon realized that I had missed taking notes on some important issues. I invited two friends that fire with wood to participate, hoping their knowledge would guide us safely through this firing. Though they had never fired this type of kiln, we managed, relying on my sketchy notes and memory to load the kiln. We proceeded to tumble stack the front of the kiln from the floor to the top and tightly stacked the remainder!  I remember being impressed with how many pots we were able to load into this kiln! We ran out of pots and even called several friends to bring more.
                                           
It became apparent, during the firing, that there was a major problem. Whenever we stoked, tremendous backpressure forced flames out of the air holes in the top of the firebox and at the person stoking. The kiln shed above the firebox became black with soot and we were afraid it would catch on fire.  We added fiber around the rafters to save the kiln shed. Later, when I called John Neely about this problem, his comment was “you did remember the triangle of death didn’t you?” (I must have not been on the shift when this was discussed.) This is the area outside the throat arch in the stacking chamber that is supposed to be left unblocked in order not to impede the flame and ash.   
 
During this firing, my husband went up on the roof while the kiln was stalled at about 2000 degrees and rebuilt the chimney.  A metal stack and cover that we had built for the top of the chimney was restricting the flow of air.  It was removed and several layers of brick raised the chimney level.  It remains today, completely open at the top.

One of my firing friends had to leave early, so we attempted to open the kiln on the second day (now I always wait three days).  As we began to unload the kiln using gloves, we began to hear the unmistakable sound of pots cracking due to the rapid cooling.  There were a few beautiful pots as a result of this firing. Many pots were lost but all in all it was an intense learning experience. Future firings have added to the list of painful lessons learned and even after 43 firings, unpredictable events still happen.  However, overall my firings are successful and I have gained a true respect for the amazing design of this kiln. Many changes have been made over the years in the stacking and firing. I still feel I learn something new with each firing. 

Some other changes I have made and problems I have encountered include the following:

One level of brick was added to the floor in the back half of the kiln. This provides a small stair-step that results in more ash and better airflow to the back of the kiln.

To extend the life of the stainless steel rods used as the grate, I attempt to rotate them at least ¼ of a turn each hour.  However, if the rods become bent, turning them may actually twist them into so use caution. Also, at the end of the firing, I remove the rods, while they are still red hot and hammer them straight so they will be ready for use in the next firing.

My recipe for wadding is ½ fireclay, ½ sand, some wood shavings and sometimes baking flour (this helps it stick together better but add flour only to wadding you will use that day.  It becomes very rancid if left sitting) plus just enough water to forms a ball. Using the correct sand for making wadding if extremely important. Medium grade sandblasting sand seems to be the best. Recently I ran out of this sand and purchased some concrete sand.  This caused the wadding to melt into a glassy pool, sticking to the pots and the shelves. Part of the kiln floor had to be replaces, the shelves sandblasted and many pots lost.

Advancer shelves, if used correctly, are wonderful in the wood kiln. I line the floor of the kiln with these lightweight shelves because they are easier to clean than the brick on the floor.  I also use these to stack pots on in the back area of the kiln.  If used in high ash areas, such as the front and middle sections of the kiln, much ash builds up on the bottom of the shelves that can drop onto the pots below, leaving unsightly marks. I found out the hard way that advancer shelves should not be stored outdoors by the kiln, especially if you live in a rainy area. During the early stages of one firing, there was a very loud explosion. Debris poured out of the chimney and filled the air. At that time I noticed there were no shelves of pots visible in the back section of the kiln.  I continued the firing since the front sections seemed to be all right and the draft flow was keeping any debris towards the back of the kiln. When I opened the kiln, the entire back section was full pottery shards and broken shelves. I realized that two advancers had exploded into dust and the largest piece of the shelves I could find was about ¼”. Other advancers were broken. Thousands of dollars of work and advancer shelves were lost.  I contacted the manufacturer and learned some interesting facts.  Regular silicon carbide shelves can absorb 12-13% moisture but are porous thus moisture can evaporate readily during a firing.  Advancers can slowly absorb only 1% moisture but, due to their density, the moisture does not evaporate easily. Because the shelves were outside between firings, they apparently absorbed this moisture that caused them to explode during the firing. At that time there were no instructions about storing the shelves.  The company agreed to replace the damaged shelves plus a few extra and now has detailed information storing and bisqueing the shelves before using.

Stacking the kiln: When stacking the kiln, with bisqued pots, I no longer abide totally by the “triangle of death” rule.  I begin by placing at least two pots that are tumble-stacked in the throat arch itself.  The first row outside the throat arch begins with pots loosely stacked on the advancer shelves on the floor. Above these pots, nine-inch posts are used for the first shelf, which is placed directly against the throat arch wall and divides the arch in half horizontally.  On this shelf I usually stack a large bottle form in the center of the shelf.  The bottle helps to split the flame as it passes through the kiln.  Tall pieces are placed on each side of the bottle, also spaced to divert the flame.  There is a second row of shelves behind this row that is stacked to the top of the kiln.  Next comes a side stoking area followed by 18 inches of shelves, a second side stoking area and, finally, 24 inches of shelves. In the side stoking areas a large pot is placed against the far wall and one or two pots wadded on their sides on the floor. Kindling size wood will be stoked on top of these pots giving them a heavy layer of rolling ash and often a beautiful blue-grey color. This area and the throat arch are prime locations in the kiln. Throughout the kiln I am alternating loose and tight stacking to direct the flame patterns.

Initially, both the porcelain and stoneware clays that I use were fired at the same time but have found that different stoking methods result in better surfaces for the different clays. I now fire the two clays separately. Refer to the firing methods described below for a description of the reducing and stoking methods for each.  

Firing Procedures for Porcelain and Stoneware Clays

1. Start warm-up gas burners after loading. Dampers out with bricks covering damper openings. Dampers remain out throughout the firing.  Chimney passives in. 4-6 firebox mouse holes open (in lower section of firebox).  Place warm-up burners in bottom two mouse holes in firebox.  I usually run warm-ups overnight.
   
2. At 6 AM, with warm-up still going, begin firing by starting to add 6-7 pieces of kindling to opening below stoking hole.  Keep same configuration stated in #1.  When temperature drops add more kindling trying to maintain 100-150 degrees an hour temperature rise.  Important to go slowly at this point in order not to break pots in the throat arch and front of the kiln. 
   
3. When temperature reaches around 800 F. remove two bricks from the top end of the firebox. Add 6-8 pieces of wood to main stoking box, criss-crossing the wood on the grates. This allows more air surface for the wood to burn. Keep burners in the bottom mouse holes. Continue stoking wood into the bottom hole.  Continue adding wood to the main firebox as the wood burns and maintain the temperature rise around 150 degrees and hour.  Around 1000 F. you may want to take warm-up burners out. The kiln usually drops in temperature at this point and it may be a struggle to keep increasing temperature but this is normal. When you remove the warm-ups, keep 2 holes open at the top and the two bottom mouse holes will each have one brick open.  Continue stoking, increasing temperature around 150-200 degrees an hour. My stoking method at this point is to stoke with 6-8 pieces after the temperature has peaked and has dropped about 20 degrees.
   
4. Cones packs are placed in the kiln at the 1st, 3rd, and 5th upper side stoke holes.  The cones are on a tray in two packs. In Stoneware firings the front pack contains cones 08, 04, 6, 8 and the second pack contains cones 9 thru 13. In Porcelain firings the first pack contains cones 014, 012, 6, 8 and the second pack contains cones 9 thru 13. 
   
5. BODY REDUCTION:  To start body reduction, close up all holes at the firebox end, remove passive dampers and open brick at damper holes.  Stuff the firebox as full as possible.  Place wood in straight, not criss-crossed, thus allowing a tighter stack and less air in the firebox (from this point on through the rest of the firing close the firebox door quickly keeping excess oxygen from entering.) Every ten minutes add more wood, keeping the firebox stuffed. There should be some smoke from the chimney but rarely black smoke.  This kiln is very efficient and can be fired with good reduction without the black smoke.  STONEWARE FIRINGS: start reduction when 04 in 1st side stoke hole starts bending.  Continue reduction for 30 minutes. Afterwards return to same stoking pattern and brick and damper configuration as before body reduction.  Then when the 04 in the last side stoke hole goes down (maybe around 2100 F) repeat body reduction as before.   PORCELAIN FIRINGS: start reduction when 012 bends in the 1st side stoke hole and continue stuffing the firebox until the 04 bends in the back peep.  This will take 4-6 hours and the temperature on the pyrometer will be around 2000 F. 
   
6. After body reduction, go back to regular stoking until cone 8 in the front bends. Brick and damper configuration should be like before body reduction.  I normally stoke every 10-15 minutes, keeping the firebox from becoming empty.
   
7. When cone 8 in the front bends I begin adding kindling in the two side stoking holes. This continues until the end of the firing. I note the temperature on the pyrometer at cone 8 (normally around 2200 F) and this is the temperature I will maintain until the next morning. By morning, cone 10 or11 are down in the front and cone 8 is down in the back.  The temperature is then raised 2225-2250 F. This will be the top temperature maintained until the end of the firing. I judge this final temperature by a combination of cone readings (which I do not feel are truly accurate in a wood firing) and the shine on the pots.  I have a metal rod I hold near the pots in the peepholes and test the reflection on the surface. I find that over-firing the kiln diminishes the colors.  Some of my best colors are often in the cooler back of the kiln. 
    
8. MAINTAINING TOP TEMPERATURE - STONEWARE FIRINGS: Ideal configuration for kiln is:  One or two peeps open at the top of firebox, 2-4 peeps out of mouse holes in bottom of firebox (depending on the coal bed size), bricks at damper holes closed or slightly open, and passives in chimney in. Stoke every ten to fifteen minutes controlling the temperature between 2225-2250 F.  Temperature can be lowered by one or more of the following: additional side-stoking (this acts like a damper and helps keep the cones in the front from bending), adding green or large pieces of wood to the firebox, closing the openings at the firebox, opening brick at damper slots, or pulling passives in chimney. PORCELAIN FIRING:  Ideal configuration for kiln is: One or no peeps open at the top of the firebox, bottom mouse holes closed or 1/2 opening on both sides, brick at damper holes open, and passives out of chimney.  Keep firebox stuffed stoking every 15 minutes.  Usually temperature drops and adjustments must be made to maintain temperature.  Increase temperature by one or more of the following: open one or two bricks out of top of firebox, open mouse holes to burn out coal bed, move brick closer or close totally at damper openings, put some or all passives in chimney. Firing this way for porcelain produces a beautiful sugary surface on the pots.

Firing is continued until 36 hours after first stoking with wood (usually about 6:00 PM on the second day).  The last stoke is a normal one and then the firebox ends are bricked up, the dampers inserted, and the passives removed. The dampers are slowly closed from both sides, backing off when flames escape from the kiln. All holes around bricks and any leaks are sealed with fire clay and sand, wet ash or clay, preventing any air leaks. When dampers are totally closed we begin reduction cooling. (At this point we usually pull the stainless steel rods out and straighten them with a sledgehammer).

1. REDUCTION COOLING.  This will last from 4-6 hours.  After kiln is closed down there will be some smoke escaping (I usually watch where the stainless steel pipes were). When the smoke goes away, a piece of kindling is inserted through either a brick opening or the pipe hole.  Smoke will appear again, and this is repeated when smoke disappears until the pyrometer reads 1750 F. This process keeps the pots and glazes reduced producing rich colors. 
   
2. OXIDATION- after reduction cooling the kiln will be oxidized for one hour.  At this time, the damper and the firebox door will be opened and the passives put back in.   After one- hour close kiln back up and remove passives and allow the kiln to cool slowly.  My kiln is usually ready to unload on the third day. On the night of the second day I raise the lid just enough to see light through it and by the next afternoon it is usually cool enough to unload. The pyrometer should read below 200 F. Remember the pots are much hotter than what the pyrometer is reading.

1. TROUBLE SHOOTING: Temperature stalling: if you cannot increase temperature and the coal bed is large (maybe from some anxious over-stoker!) remove some of the mouse hole brick from the bottom of the firebox and let the coal bed burn down. I sometimes use a poker to open up air spaces in the mouse holes. You may lose temperature but it will be easily regained after the coals burn down. Wet or green wood will also cause a stalling out of the temperature.

Back of kiln not getting hot enough: you can soak the kiln by pulling the passives in the chimney and opening brick at damper slots. You can also close up some air at the firebox end, or do more side stoking in the back.

These are some of the techniques that I have found helpful in creating the surfaces on the clay I desire.  Many factors play a part in this including my clays, altitude, weather and wood (a combination of white pine, oak, hickory, maple, and apple).  I am sure there are hundreds of other factors that affect the outcome of a firing but hopefully some of these Painful Lessons Learned will help make your firing in the train better and more understandable.